A step-through debugger for GHC Haskell.
Haskell Debugger
We are working on a first class debugger for Haskell!
Installation
Please find up to date installation instructions on the project homepage!
[!WARNING]
hdbcan currently be compiled with the 9.14 alpha pre-releases or with a nightly version The first release it will be compatible with is GHC 9.14.
To install and use the debugger, you need the executable hdb and the VSCode extension Haskell Debugger.
Since hdb implements the Debug Adapter Protocol (DAP), it also supports debugging with tools such as vim, neovim, or emacs -- as long as a DAP client is installed and the launch arguments for hdb configured.
To run the debugger, the same version of GHC which compiled it needs to be in PATH. Make sure the DAP client knows this. For instance, to launch VSCode with a specific GHC use:
PATH=/path/to/ghc-dir:$PATH code /path/to/proj
Usage
To use the debugger in VSCode, select the debugger tab, select Haskell Debugger, and create a launch.json file by clicking the debugger settings icon (next to the green run button). Now, it is also supported to just Run a file which contains a main function.
The launch.json file contains some settings about the debugger session here. Namely:
| Setting | Description |
|---|---|
projectRoot | the full path to the project root. this is typically ${workspaceFolder}, a value which is interpolated by the editor with the actual path |
entryFile | the relative path from the project root to the file with the entry point for execution |
entryPoint | the name of the function that is called to start execution |
entryArgs | the arguments passed to the entryPoint. If the entryPoint is main, these arguments are passed as environment arguments (as in getArgs) rather than direct function arguments. |
extraGhcArgs | additional flags to pass to the ghc invocation that loads the program for debugging. |
Change them accordingly.
To run the debugger, simply hit the green run button. See the Features section below for what is currently supported.
Multiple home units session
Multiple home units is supported but currently may require a workaround (issue is tracked by #38).
If your multiple home units session does not work by default (e.g. if you cannot set breakpoints on different units), and you do not have a hie.yaml file, you may want to try creating a hie.yaml file in the root of the workspace with:
cradle:
cabal:
component: "all"
Related Work
hdb is inspired by the original haskell-debug-adapter by @phoityne.
hdb improves on the original ideas implemented in haskell-debug-adapter but makes them more robust by implementing the debugger directly via the GHC API (similarly to HLS), rather than by communicating with a custom ghci process.
We have been doing custom work on GHC to support debugging in a predictable, robust, and more performant way. That is why hdb is only compatible with the latest and greatest GHC. If you want to debug using an older GHC version (9.12 and older), please check out haskell-debug-adapter.
To implement the Debug Adapter Protocol (DAP) server part, we are using the dap library by @dmjio. dap is a framework for building language-agnostic DAP.
The hdb is transparently compatible with most projects (simple, Cabal, Stack, custom hie.yaml) because it uses hie-bios to figure out the right flags to prepare the GHC session with.
Features
Many not listed! Here are a few things:
Stepping
- [x] Continue (resume execution forward)
- [x] Next (step within local function)
- [x] Step into (single step to next immediate tick)
- [x] Step out (execute until end of function and break after the call)
In Reverse
- [ ] Local step backwards (ie reverse of Next)
- [ ] Single step backwards (ie reverse of Step into)
- [ ] Continue backwards (resume execution backwards until a breakpoint is hit)
Breakpoints
- [x] Module breakpoints
- [x] Function breakpoints
- [x] Exception breakpoints
- [ ] Data breakpoints
- [ ] Instruction breakpoints
Conditionals
- [x] Conditional breakpoints (breakpoint is hit only if condition is satisfied)
- [x] Hit conditional breakpoints (stop after N number of hits)
Custom Debug Visualizations
The user can extend the debugger visualization behavior (in a plugin sort of way) by implementing DebugView from haskell-debugger-view for the desired types.
We ship built-in custom instances for various base datatypes, such as String and (a, b), and for a core packages as well, such as Text, ByteString, Map and IntMap.
Here is an example of a completely custom DebugView instance for a user-defined datatype. You can also see how the IntMap is displayed as a pair of Int keys to their values as opposed as as a Bin/Tip tree mimicking its real definition:
When the package dependency closure includes haskell-debugger-view, we will use that unit specifically. When it is not in the dependencies, we will load a built-in version in memory.
Talks
MuniHac 2025: A modern step-through debugger for Haskell
ZuriHac 2025: Haskell Implementor's Workshop: The GHC Debugger
Building from source
To build hdb:
cabal build -w /path/to/recent/ghc exe:hdb
To build the VSCode extension
cd vscode-extension
nix-build
Testing
cd test/integration-tests
make GHC=/path/to/recent/ghc \
DEBUGGER=$(cd ../.. && cabal list-bin -w /path/to/recent/ghc exe:hdb)